1. Field of Invention
The present invention concerns a diecasting machine for production of mouldings from thixotropic metal billets, containing a sprue system which connects a cylindrical casting chamber cavity with a moulding cavity, where the sprue system has a cylindrical sprue cavity immediately adjacent to the casting chamber cavity and contains at least one sprue, and all sprues lead laterally away from the generated surface of the sprue cavity, and each sprue has a concentric center line and at its end facing towards the moulding cavity has an inlet opening for introduction of the thixotropic metal alloy into the moulding cavity, and the sprue system is connected to the casting chamber cavity by a passage perpendicular in relation to a concentric longitudinal axis of the cylindrical casting chamber cavity, and the inlet openings are arranged in relation to the passage opening such that the surface normals of the inlet openings do not coincide with the longitudinal axis of the cylindrical casting chamber cavity.
2. Background and Prior Art
Diecasting machines for production of mouldings from thixotropic metal billets are known in themselves. Such diecasting plants essentially contain a casting chamber to hold the diecasting alloy or thixotropic metal billet, a ram moving in the longitudinal direction in the casting chamber for applying pressure to the diecasting alloy or thixotropic metal billet, at the end of the casting chamber opposite the ram a casting chamber opening, and a sprue system comprising essentially a sprue to transfer the diecasting alloy or thixotropic alloy paste from the casting chamber opening into the moulding cavity.
EP-A 0 718 059 describes a horizontal diecasting machine for production of mouldings from a thixotropic alloy paste, where the diecasting machine has an oxide scraper which is located between a semi-cylindrical area of the casting chamber, suitable for insertion of a thixotropic metal billet, and the moulding cavity, and which serves to prevent oxide inclusions in the alloy structure of the moulding.
DE-OS 40 15 174 describes a diecasting machine with a two-part mould for casting of plastic or metal, where between the two mould halves is fitted a specially shaped casting holding device which can assume a changing passage cross section and in its closing position delimits a tapering cross section which is smaller than the predetermined cross section of the casting chamber opening.
The process for production of mouldings from thixotropic i.e. part solid/part liquid metal billets is known as thixoforming. The metal billets are all billets of a metal which can be transformed into a thixotropic state. In particular the metal billets can consist of aluminium, magnesium or zinc or alloys of these metals.
Thixoforming utilizes the thixotropic properties of part liquid and part solid metal alloys. The phrase xe2x80x9cthixotropic behaviour of a metal alloyxe2x80x9d means that a correspondingly prepared metal behaves as a solid when not under load, but under a thrust load its viscosity reduces to the extent that it behaves in a similar way to a metal melt. This requires heating of the alloy in the setting interval between the liquid and solid temperature. The temperature must be set such that for example a structure proportion of 20 to 80 w. % is melted but the rest remains in solid form.
In thixoforming, part solid/part liquid metals are processed into mouldings in a modified diecasting machine. The diecasting machines used for the thixoforming differ in relation to diecasting machines for diecasting metal melts for example by a longer casting chamber to hold the thixotropic metal billet and a larger ram stroke required as a result, and for example a mechanically reinforced design of the parts of the casting machine guiding the thixotropic metal alloy due to the higher pressure loading of these parts during thixoforming.
Thixoforming takes place for example with a horizontal diecasting machine. In this machine the casting chamber which holds the thixotropic metal billet lies horizontal. In thixoforming a thixotropic metal billet is inserted in such a horizontal casting chamber of a diecasting machine and, by application of pressure from a casting ram, is introduced at high speed and under high pressure into a casting mould usually consisting of steel, in particular hot worked steel, i.e. it is introduced or injected into the moulding cavity of the casting mould where the thixotropic metal alloy sets. The pressure applied to the thixotropic metal billet is typically 200 to 1500 bar and in particular between 500 and 1000 bar. The resulting flow speed of the thixotropic alloy paste is for example 0.2 to 3 m/s and in particular 0.3 to 2 m/s.
The casting structure forming during setting of the thixotropic metal alloy in the casting mould essentially determines the properties of the moulding. The structure formation is characterised by the phases such as mixed crystal and eutectic phases, the casting grains such as globulites and dendrites, segregations and structure faults such as porosity (gas pores, micropores) and contamination, for example oxides.
The metal billets used for thixoforming of part solid alloys have a process-induced fine grain whichxe2x80x94if no grain coarsening occurs during pretreatment of the thixotropic metal billets i.e. during heating of the billets and their transport into the diecasting machinexe2x80x94recurs in the alloy structure of the mouldings. A fine grain generally improves the material properties, increases the homogeneity of the alloy structure and helps avoid structural defects in the moulding. Thixoforming of part solid alloys in comparison with diecasting of metal melts also has further substantial advantages. These include a significant energy saving and shorter production times as firstly the thixotropic metal billets, in comparison with diecasting of metal melts, need be heated to a lower temperature and thus for a shorter time before thixoforming, and secondly, in the casting mould they cool or return to a solid state more quickly, which contributes to a reduction in grain coarsening. The energy saving arises in particular because a majority of the melt heat and the entire superheating heat, i.e. the heat additionally supplied to the metal alloy to achieve a temperature increase above the melt point to ensure the liquid state of the metal alloy, and the energy for keeping the melt warm, are no longer required. A further advantage is also the better dimensional precision due to the lower shrinkage and production of mouldings close to the final dimensions, whereby the machining steps are reduced and alloy material saved. Also, the processing temperature is around 100xc2x0 C. lower and reduces the temperature change stress on the individual components of the diecasting machine, which extends the tool life. The lower processing temperature in thixoforming than in diecasting of metal melts allows the processing of alloys with a low iron content, as no alloying of the tool from contact melting occurs. Thixoforming also allows a better mould filling with fewer air inclusions.
In diecasting machines which are known from the state of the art, a metal billet in the thixotropic state, usually a thixotropic aluminium billet, is inserted into a casting chamber (or more precisely into a casting chamber cavity inside the casting chamber) and by means of pressure application is pressed through a usually cylindrical constriction at one end of the casting chamber known as the passage opening. The thixotropic material is thus sheared. The sheared thixotropic material, starting from a sprue cavity lying next to the passage opening, is deflected into trapezoid sprues and reaches the moulding cavity of a mould. Normally the sprues are arranged at approximately right angles to the concentric center axis of the passage opening. The arrangement between the casting chamber and moulding cavity is referred to below as the sprue system. The sprue system is used to introduce the thixotropic alloy paste in the casting chamber into the moulding cavity of the casting mould.
The mechanical stress on the thixotropic alloy paste during its transfer from the casting chamber cavity to the moulding cavity causes a shear liquefaction of the thixotropic alloy i.e. the thixotropic alloy becomes more liquid as a result.
The following requirements are imposed on a sprue system for thixoforming:
a) Good filling behaviour: the sprue system must be filled as evenly as possible over its entire cross section. In the speed range of the thixotropic alloy used, no gas or oxide inclusions may occur.
b) Good flow behaviour: the flow must be as laminar as possible to avoid eddying and undesirable liquefaction of the thixotropic material.
c) Good shearing behaviour: the shear liquefaction must be as homogeneous as possible over the entire cross section and the shear liquefaction must be kept as low as possible.
d) Low heat loss: on its passage through the sprue system, the thixotropic material should lose as little thermal energy as possible.
e) Minimum volume of sprue system: the material remaining in the sprue system at the end of the thixoforming process is not used for the filling process of the moulding cavity. Therefore the sprue system should have a minimum volume to guarantee optimum output of thixotropic material into the moulding cavity.
f) Good addition behaviour: during setting of the moulding, the thixotropic material in the sprue system must remain cohesive and liquid so that firstly the pressure transfer from the casting ram to the moulding can be maintained and secondly the volume deficit on the moulding caused by setting-induced shrinkage can be compensated by the addition of thixotropic material.
g) Good pressure transfer: the sprue system should allow as low a pressure loss as possible between the casting chamber cavity and the moulding cavity.
Sprue systems which are known from the state of the art only fulfil these requirements in part. In particular, the known sprue systems have too great a volume so that the output of thixotropic material per moulding can be improved substantially. Too great a volume of the sprue system used, in particular reduces the economic efficiency of the process.
Another disadvantage of the known sprue systems concerns the speed-dependent filling behaviour. The filling behaviour of a sprue system can differ widely depending on the ram speed and starting condition of the thixotropic billet. Thus at high ram speeds for example undesirable air inclusions can occur in the thixotropic alloy paste of the sprue system. On very rapid mould filling during thixoforming, turbulent flow conditions can occur which can lead to gas inclusions (air, separating agent or lubricant) in the mould, whereby any desirable subsequent heat treatment, for example solution heat treatment of the moulding, is often rendered impossible. Gas inclusions close to the surface of the moulding can for example lead to undesirable blister formation during solution heat treatment due to the high gas pressure.
Another disadvantage of the known sprue system concerns the uneven flow behaviour. The flow established during thixoforming after filling the sprue system with thixotropic material is uneven in most cases. It has been found in particular that sudden direction changes and/or changing cross section ratios lead to local speed changes of the thixotropic material. It has also been found that with angular cross sections of the sprues only part of the available cross section is effectively used for guiding the thixotropic material.
In view of the disadvantages described above of the known sprue systems for diecasting machines for production of mouldings from thixotropic material, the inventors have faced the task of preparing a sprue system which avoids the said disadvantages and which fulfils optimally the requirements imposed for a sprue system of a diecasting machine for thixoforming.
According to the invention, this is solved in that each sprue has a circular or elliptical cross section with a substantially constant cross sectional area over its entire length, and immediately next to the sprue cavity has a manifold, where the part of the sprue between the manifold and the inlet opening describes a straight tubular channel section and the manifold is formed such that its center line has a constant bending radius, and a tangent to the center line continued to the passage opening with the same bending radius at the passage opening runs parallel to the longitudinal axis of the cylindrical casting chamber cavity, and a tangent to the center line at the end of the manifold facing towards the inlet opening coincides with the center line of the straight tubular channel section.
By the design of the sprue system according to the invention, direction changes and the associated shear liquefaction of the thixotropic alloy paste during transport from the passage opening to the inlet opening remain minimal. Each sprue preferably has a constant cross sectional area between the sprue cavity and the inlet opening. This keeps the flow speed of the thixotropic alloy as constant as possible and minimises the shear effect on the thixotropic alloy.
Also, preferably the sum of the cross sectional surfaces of the individual sprues substantially corresponds to the cross sectional area of the passage opening. The sum of the cross sectional areas of the individual sprues next to the sprue cavity, in a particularly preferred form, deviates by no more than xc2x110% from the cross sectional area of the passage opening.
In a further preferred embodiment of the sprue system, the sprue has at its end facing against the moulding cavity a chamfer area which ends in the corresponding inlet opening. Preferably, the sprues between the sprue cavity and the relevant chamfer area have a tubular channel section with a circular cross section and constant radius. The channel section between the sprue cavity and the chamfer area firstly concerns the manifold and secondly the straight channel section between the manifold and the chamfer area of each sprue. This circular cross section minimises the ratio of surface area to volume. Also, the circular cross section allows full utilisation of the available channel cross section.
Preferably, the inlet openings have an elliptical cross section. The inlet opening arises from the plane of section of the chamfer area of the sprue with the widening moulding produced in the moulding cavity. With a flat moulding wall, an elliptical inlet opening therefore arises. With curved moulding geometries normally more complex planes of section occur.
The chamfer area constitutes a channel-like transition area between the straight section of the sprue with circular cross section and the inlet opening. Preferably, the chamfer area along its center line has a cross section which gradually transforms from a circular to an ever flatter elliptical cross section, where this transitional area ends in an elliptical cross section corresponding to the inlet opening. Preferably, in the chamfer area the cross sectional area is kept substantially constant in size where changes of cross sectional area of up to 30% in size are included; in particular the cross section of the chamfer area along its center line can gradually expand or contract slightly.
In a further preferred embodiment the sprue system according to the invention has a catchment pocket for the surface oxide layer of the thixotropic metal billet. During pretreatment, storage and the heating process of the thixotropic metal billet, a metal oxide layer normally occurs. To avoid inclusion of such oxidic constituents in the alloy structure of the moulding, the oxidic generated surface of the thixotropic metal billet is removed usually before or in the casting chamber. Normally, an oxide layer remains on the face of the thixotropic billet. The catchment pocket proposed in the embodiment of the sprue system according to the invention thus allows the deposit of this surface oxide layer in a flow-mechanically dead zone at the end of the sprue cavity remote from the passage opening. The catchment pocket is for example formed by a cylindrical protuberance of the sprue cavity on the side remote from the passage opening.
The sprue system according to the invention is preferably used for horizontal diecasting machines.
Also, preferably the straight channel sections of the sprues run perpendicular to the longitudinal axis of the casting chamber cavity. The bending radius of the center line of the sprue manifold corresponds to the distance of the passage opening from a straight line containing the center line of the straight tubular channel section of the corresponding sprue.
According to the invention the bending radius of a center line in the manifold area is determined for example by the intersection point of the angle bisector between the longitudinal axis of the casting chamber cavity and the center line of the straight part section of the corresponding sprue with a plane through the passage opening, where the distance between this intersection and the center point of the passage opening gives the bending radius Rk.
The transition between the casting chamber cavity and sprue cavity can be sharp-edged or rounded. In a sharp-edged design, this transition is described by the passage opening. Preferably, however, a rounded transition is used. Here the passage opening is described by the point at which the cross section is at its smallest or where the cross section assumes a constant value i.e. transforms into a sprue cavity with constant cross section. In the rounded design form of the transition between the cylindrical casting chamber cavity and the passage opening therefore a transitional area is formed with a constantly reducing cross section. The creation of such a transitional area causes an even shear effect of the thixotropic alloy paste. This also avoids the break-away of the thixotropic alloy flow from the wall of the passage opening as frequently occurs with sharp-edged transitions and high flow speeds.
Further advantageous designs of the sprue system according to the invention arise from the dependent claims.
The sprue system according to the invention is primarily suited for thixoforming of all metal alloys which can be transferred to a thixotropic state. Preferably, the sprue system according to the invention is used for thixoforming of aluminium, magnesium or zinc alloys. Particularly preferably, the sprue system according to the invention is suitable for thixoforming of aluminium diecasting alloys, in particular AlSi, AlSiMg, AlSiCu, AlMg, AlCuTi and AlCuZnMg alloys.
The sprue system of the invention has the following advantages over the state of the art:
a) Minimum sprue system volume:
By the use of round sprues, the total surface is kept as small as possible. Also, because of the optimum ratio of surface area to volume, the heat loss is minimal. Therefore less thixotropic material is required to compensate for the heat loss of the thixotropic alloy paste in the sprue system.
b) Good filling behaviour:
The filling behaviour of the sprue system is very good in the mould filling speed rangexe2x80x94i.e. the flow speed of the thixotropic alloyxe2x80x94normally used for thixoforming, i.e. no air inclusions occur even at relatively high flow speeds.
c) Flow behaviour:
The flow behaviour with a sprue already filled with thixotropic alloy is excellent as the entire cross sectional surface of the sprue is utilised and no flow-mechanically dead zones occur. Also, the round channel cross section of the sprue allows the formation of a laminar flow for the entire speed range used for mould filling.
d) Adjustability of viscosity:
Due to the low shear liquefaction of the thixotropic alloy at the passage opening and the sprue, a high viscosity of the thixotropic alloy can be retained as far as the inlet opening. At the inlet opening the viscosity of the thixotropic alloy paste required for filling the moulding cavity can be set.
e) Minimum pressure loss and good addition behaviour:
The ram pressure is transferred extremely well by the curved inlet channels according to the invention i.e. the pressure loss in the sprues is minimal and because of the hydrostatic pressure, is determined in particular by the selected height of the corresponding inlet opening. The addition behaviour is also substantially determined by the height of the inlet openings due to the low pressure drop in the sprues.
Diecasting machine with a horizontal casting chamber in which the transition from the casting chamber cavity to the sprue cavity is sharp-edged, and the sprue system has two sprues of the same dimensions each with one chamfer area. The cross section of the sprue section between the sprue cavity and the chamfer area is circular and has a diameter of 2 R=25 mm. The bending radius of the manifold is 42.5 mm. The passage opening diameter is 35 mm. The sprue cavity is cylindrical and has a horizontal concentric longitudinal axis which also coincides with the concentric longitudinal axis of the casting chamber cavity. The sprue cavity has a diameter of 35 mm. The length of the sprue cavity is such that between the two manifolds a catchment pocket is formed for the surface oxides of the thixotropic billets, where the cross sectional dimensions of the catchment pocket correspond to those of the sprue cavity. The straight channel section of each sprue lies vertical and thus perpendicular to the concentric longitudinal axis of the casting chamber cavity, where the one sprue extends vertically downwards and the other sprue leads vertically upwards. The height of the start of the chamfer area, measured from the concentric longitudinal axis of the sprue cavity, amounts to 102.5 mm. The length of the chamfer area is 50 mm. The inlet openings lie in a horizontal plane and have an ellipsoid form with a main axis length a and a secondary axis length b. The shape of the chamfer area can be described in a Cartesian co-ordinate system in which the x axis lies parallel to the concentric longitudinal axis of the casting chamber cavity, the y axis parallel to a vertical, and the z axis also lies in a horizontal plane through the x axis such that:
x(y)=(bxe2x88x92R)xc2x7y/c+R
and
z(y)=(cxc2x7R2)/(bxc2x7yxe2x88x92Rxc2x7y+Rxc2x7c)
where R is the constant radius of the circular cross section sprue section between the sprue cavity and the chamfer area, b the length of the secondary axis of the inlet opening and c the length or height of the chamfer area. In this Cartesian co-ordinate system the main axis a of the inlet opening lies parallel to the z axis and the secondary axis b parallel to the x axis. The inlet openings thus have an ellipse shape with a secondary axis diameter of 2 b=6 mm and a main axis diameter of 2 a.